All amp models are 2 slot entities in the new scheme, and a cabinet is 1 slot, so for full amp modeling with cab sims it is a minimum of 3 slots, leaving a max of 4 slots for everything else. IMO, best use of the B3n is for either: all front end effects taking advantage of the 7 available slots - or - as a back end amp/cab sim with post processing (EQ, noise reduction, compression) with other separate pedals providing front end effects. A bass version of the G5n would be great to cover all the bases, but Zoom is not releasing one.
Another strategy is to use a single slot "drive" pedal for preamp tone (limited, though, in terms of tone/gain control) and then use a cab block to get the flavor of the amp, since much of a bass amp's tone is due to the cabinet itself, perhaps even moreso than the preamp in many cases.
Remember that Zoom is going to provide additions to the B3n in the form of downloads via Guitar Lab software. GK and Hartke are icons in the bass industry, so I'd be shocked if ZOOM didn't offer models of their products at some point in time, hopefully sooner rather than later. I'm not a betting man, but I'd bet on that. I have no way of knowing this for a fact, but man it would be a brutally poor strategy to NOT have GK/Hartke in the stable.
Zoom is known to respond to such questions via FaceBook, so maybe you can pop the question there.